Casting high-strength aluminum alloys



nited States Patent 3,485,681 CASTING HIGH-STRENGTH ALUMINUM ALLOYS Herbert Greenewald, 313, Columbus, Ohio, assignor to North American Rockwell Corporation, a corporation of Delaware Filed Feb. 9, 1967, Ser. No. 614,837 Int. Cl. C2141 7/13, 1/00 US. Cl. 1483 14 Claims ABSTRACT OF THE DISCLOSURE SUMMARY OF THE INVENTION Aluminum alloys, typically having strength alloying elements in the range of approximately 5.0% to 12.8% by weight selected from the group consisting of copper, magnesium, and zinc and having less than approximately 0.5% by weight of silicon, are sequentially: (1) melted in a non-reactive environment such as argon having fewer than 100 parts per million (p.p.m.) by weight of hydrogen water, or hydrocarbons and poured in that environment into a non-reactive mold heated to a temperature above the alloy liquidus curve; (2) solidified in the mold and in said environment to a temperature below the alloy solidus curve but above approximately 600 F. preferred and 500 F. in any event by progressively cooling the mold and cast alloy slowly from those mold/alloy regions distant-most from the casting riser to those mold/alloy regions at the casting riser; (3) cooled to an ambient temperature after intermediate overaging or intermediate casting/mold separation; and (4) heat-treated at conventional solution and homogenizing temperatures and times but in a nonreactive environment such as argon having fewer than 100 ppm. of hydrogen, water, or hydrocarbons cooled from the heat-treating temperature to ambient temperature at a rate sufficient to obtain a non-equilibrium metallurgical structure, and aged at conventional aging temperatures and times to obtain optimum metallurgical properties. Defect-free castings having configurations with substantial thin-walled sections and with shape factors significantly greater than approximately 100 have been manufactured from high-strength aluminum alloy in accordance with the invention to obtain throughout minimum ultimate tensile strength of approximately 76,000 p.s.i., minimum yield tensile strengths (0.2% offset) of approximately 70,000 p.s.i., and minimum elongations (in 2") of approximately 2.0%, each in an as-cast, heattreated, and aged condition.

DESCRIPTION OF THE DRAWINGS FIGS. 1(a) and 1(1)) provide a sequential flow diagram of the method of this invention and also identify particular limitations relating to the processing of a specific class of high-strength aluminum alloys.

3,485,681 Patented Dec. 23, 1969 ice DETAILED DESCRIPTION The instant invention relates to the processing of highstrength aluminum alloys, such alloys typically containing strength alloying elements in the range of approximately 5.0% to 13.0% by weight selected from the group consisting of copper, magnesium, and zinc. Representative alloys of this class include the conventional wrought aluminum alloys having 7075, 7079, 7178, and 2024 numerical identifications; also included are such alloys with modifications involving the addition of titanium and beryllium in specified amounts as grain refiner ingredients and involving the restriction of impurities, including silicon and iron, to levels below approximately 0.25% by weight maximum. Examples of modified 7000-series high-strength aluminum alloys that have been used in the practice of this invention are given in the detailed description of copending application Ser. No. 614,838 filed the same day as this application and covering an invention also assigned to the assignee of this invention but entitled Aluminum Casting Alloy.

It should also be noted that the method of this invention is particularly significant with respect to casting configurations having a substantial total area of thin-walled sections. As used herein, a thin-walled casting or section is defined as involving a shape factor in excess of approximately 100. Such shape factor, in turn, is defined in accordance with conventional practice as being the quotient of the sum of the casting developed plan length and the casting developed plan width divided by the casting average thickness. An aircraft box beam having a developed plan area with overall dimensions of approximately 12 32 and having an average thickness of 0.060" was determined to have a shape factor of 733. In general, the instant invention has application to casting configurations wherein shape factors in the range of from approximately to at least 8,000 are involved.

As suggested by FIG. 1, the procedure of this invention for processing high-strength aluminum alloy composition to obtain sound castings, particularly in thin-walled configurations with increased tensile strength properties throughout in the as-cast, heat-treated, and aged condition, is essentially comprised of the steps numerically designated 11 through 23. The aging and post-aging cooling steps referenced as 22 and 23 in the process flow diagram are entirely conventional. However, important differences over conventional practices exist with respect to the steps designated 11 through 21. It should be noted that alternate sequences are available with respect to accomplishing the steps sequentially intermediate blocks 13 (First-Stage Cooling) and 20 (Solution and Homogenization Heat-Treating). Different of the alternate sequences may be preferred depending on hereinafter-described time limitations; in any event, the desired end results are obtained regardless of which of the disclosed alternate sequencies is selected.

Throughout the following detailed description of the invention, frequent reference is made to a required controlled atmosphere and a required controlled pressure. As used herein, controlled atmosphere means an environment that is non-reactive with respect to the high-strength aluminum alloy being processed and that further is hydrogen-free as well as moisturehydrocarbon-free. Pure argon having less than 100 parts per million (p.p.m.)

air or an oxygen-equivalent medium maintained above the minimum removal temperature.

If either step 17 or 18 is selected for alloy processing immediately following first-stage cooling, the subsequent second-stage cooling is accomplished as shown by block 19. Such post-solidification cooling may be conveniently accomplished in ambient atmospheres at ambient pressures. The cooling starting temperature for the removed alloy casting is in the previously stated preferred range of 600 F. to 800 F. and the final temperature is ambient (room) temperature. Cooling rates are normally those obtained by cooling the removed casting by immersion in the ambient temperature atmosphere.

If casting removal cannot be accomplished at elevated temperatures as in connection with step 17 or 18, over aging step 14 is selected to follow first-stage cooling (13). In the over aging operation the casting and mold combination is maintained at a temperature normally in the range of 500 F. to 600 F. for sufiicient time to develop increased ductility in the casting. For 7000-series alloy compositions, heating for 5 hours at 500 F. subsequent to first-stage cooling is normally adequate. Ambient atmospheres and ambient pressures may be utilized.

After over aging step 14 is completed, second-stage cooling in the alternate sequence is accomplished as indicated by block 15 and the casting subsequently removed from the mold as indicated by block 16. Second-stage cooling step 15 corresponds to second-stage cooling step 19 with respect to absence of critical limitations except that the starting temperature is in the range of 500 F. to 600 F. rather than 600 F. to 800 F. The removal of the completed casting from the mold is accomplished in connection with step 16 by forceful ejection in a conventional manner or by disintegration of the mold.

It is also possible, with proper equipment, to eliminate steps 14 through 16 and 19 from use in connection With the instant invention. More particularly, and as shown by FIG. 1( b), the hereinafter-described solution and homogenization heat'treating step of block may be initiated and accomplished immediately after step 17 or step 18 is completed.

in order to develop optimum strength characteristics in the solidified casting, the high-strength aluminum alloy should be heat-treated and aged after removal from the mold. Heat-treating is accomplished essentially in accordance with the critical limitations specified with block 20. It is important that such heat-treating be accomplished in an environment with a controlled atmosphere and, in the case of 7000-series wrought aluminum alloys, With a pressure above the prescribed minimum pressure of 15" of mercury (absolute). Time and temperature parameters, however, are essentially conventional; 72 hours at 920 F. is generally satisfactory. Time periods to as little as 20 hours have sometimes been used. Generally, the longer time periods are preferred if optimum strength characteristics are to be obained. The step is accomplished so as to eliminate second-phase precipitates at alloy grain boundaries.

The post-heat-treat cooling step 21 preferably occurs at ambient temperatures. Since the cooling rate must be sufficiently high to obtain a non-equilibrium structure, the cooling rate must involve a comparatively high rate Of heat transfer. As indicated in FIG. 1, it is preferred that the completed heat-treated casting be quenched from the solution and homogenization heat-treating temperature to ambient temperature in ambient temperature water. This diifers from the conventional practice involving postheat-treat cooling using heated water such as 180 F.

The steps identified by blocks 22 and 23 are conventional accelerated aging steps. Temperature-time histories in the range of from 450 F. for .2 hours to 250 F. for 24 hours are generally adequate. In the case of 2024 alloy, however, aging may require a temperature of 375 F. for 24 hours. No environmental controlled atmosphere or environmental controlled pressure is required. Cooling in accordance with step 23 is conventional and may be accomplished by immersing the acceleration-aged alloy in the ambient temperature environmental atmosphere.

Aluminum alloy castings having thin-walled sections (e.g., 0.060 thickness) over substantial areas have been made in accordance with the invention described herein. The alloys actually used involved the compositions set forth in the following Table I:

Tensile strength and elongation property tests were performed in a conventional manner with respect to such castings. The obtained data is set forth in the following Table 11:

TABLE II Melt:

Property 7-5 F J K Ultimate Tensile Strength, p.s.i 79,100 76,000 78,000 84,500 Y181d Tensile Strength (0.2% offset),

p.s.1 72, 900 68,800 78, 400 Elongation (in 2), percent 2.1 1.9 7. 3 4. 2

All of the specimens were tested at room temperatures in the as-cast condition. Also, all of the specimens were processed in accordance with the sequence of steps 11 through 16 and 20 through 23. Different specific times and temperatures, however, were utilized in connection with the solution and homogenization heat-treating of step 20 and the aging of step 22. More specifically, cast sections of the Melt 7-5 material were solution heat-treated 19.5 hours at 880 F. and then 24.0 hours at 920 F.; the Melt 7-5 specimens were aged for 24 hours at 300 F. Intermediate cooling was by water quenching to room temperature from 920 F. With respect to Melt I and Melt K specimens, the time and temperature for solution and homogenization heat-treating was 72 hours at 920 F. Each of the Melt I and Melt K specimens were acceleration aged at 250 F. for 24 hours, and intermediate cooling was by water quenching to room (ambient) temperature. Melt F specimens tested and reported above were solution and homogenization heat-treated in air for 8 hours at 880 F. followed by 12 hours at 920 F.; the low elongation value and below-minimum yield tensile strength are attributable, at least in part, to the failure to adhere to the stated processing critical limitations as to atmosphere conditions.

I claim:

1. In a method of processing an aluminum alloy having elements selected from the group consisting of zinc, copper, and magnesium in the range of approximately 5.0% to 13.0% total by weight into a casting prior to solid solutioning and homogenization heat-treating followed by quenching, the steps. of:

(a) melting said alloy to a temperature in the range of 1250 F. to 1350 F.;

(b) pouring said melted alloy into a non-reactive casting mold that is heated to a temperature in the range of approximately 1200 F. to 1350 F. and that has an interior cavity with a casting configuration of shape factor greater than approximately and an attached riser configuration;

(c) solidifying said poured alloy into a casting direcsaid melting, pouring, and solidifying steps each being accomplished in an oxygen-free, non-reactive environment having fewer than 100 ppm. of hydrogen, water, and hydrocarbons.

13. The invention defined by claim 12, wherein said oxygen-free, non-reactive environments are each pure argon.

14. The invention defined by claim 12, wherein said oxygen-free, non-reactive environments each have an absolute pressure greater than approximately 15 of mercury.

1 0 References Cited UNITED STATES PATENTS 4/1939 Goetzel 148-159 11/1941 Bonsack 1483 OSCAR R. VERTIZ, Primary Examiner H. S. MILLER, Assistant Examiner US. Cl. X.R. 148-459; 164322 

